This invention relates to a process for extraction and dehalogenation of halogenated aromatic hydrocarbons. More specifically, it relates to a process for extracting and dechlorinating polychlorinated biphenyls (PCB's).
Polychlorinated biphenyls are a group of several hundred organic compounds which have a basic structure of two benzene rings joined by a single (sigma) bond to form a biphenyl. Chlorine atoms are substituted for some of the hydrogen atoms of the benzene rings. Commercial mixtures of PCB's generally contain 40 to 60 percent chlorine and have as many as 50 detectable isomers. Mixtures of PCB's are extremely resistant to degradation; are thermally stable; are resistant to oxidation; are resistant to reaction with acids, bases and other chemicals; and are good electrical insulators. PCB's also are very effective fire retardants and have been used as such in fluids which operate at high temperatures and in electrical transformers and capacitors.
A problem associated with the use of PCB's is that they are toxic organic chemicals. It has been found that PCB's tend to remain in the fatty tissues of an organism once entry has been gained and eventually can accumulate to toxic levels. For this reason, the sole U.S. manufacturer of PCB's has stopped its production.
More than 90 percent of all PCB's produced were used in electrical devices-mainly transformers and capacitors. Under existing federal regulations, PCB's still in use eventually must be removed from service and destroyed. Electric utilities, who own the majority of the nation's PCB inventory, are very interested in finding an inexpensive and environmentally acceptable method of destroying PCB's. Unfortunately, because of their recalcitrance to reaction, and thermal stability, PCB's are extremely difficult to safely and economically destroy. It is particularly difficult to remove PCB's from askarel transformers, which have very high concentrations of PCB's even after the fluid in the transformer has been cleaned or replaced, because PCB's continue to be leached out of the transformer components and recontaminate the fluid. Thus, these transformers have to be cleaned by a process which will continue to remove PCB's from the fluid for a period of days or weeks which adds to the expense of such process.
One of the safest and environmentally acceptable methods of destroying PCB's is incineration. However, incinerators are expensive to operate. Additionally, there are high insurance, transportation, and fuel costs associated with incineration. Public opposition to the transportation and processing of toxic materials also makes incineration unpopular.
There are a number of chemical processes for destroying PCB's. One process which is in commercial use involves reductive dechlorination which is used for mineral oil containing PCB's and renders the PCB's inert while alowing salvage of the mineral oil carrier. A disadvantage of the process is that it is most suitable for processing large quantities of contaminated oil (i.e. quantities in excess of 380 liters) and is not easily scaled down to adapt it to process the small quantities of oil (i.e. on the order of 76 to 152 liters) which are typically found in electrical transformers. Additionally, the equipment for the process is expensive and economically cannot be left in place for the length of time required to process the small quantities of material found in electrical transformers. Furthermore, the process is limited to processing oil having PCB concentrations of less than 10,000 parts per million (ppm) (1%).